The present invention relates to a rectifier of the type in which a semiconductor active element such as a transistor is used as a rectifying element.
In general, rectification losses are dependent upon the characters or properties of not only rectifying elements themselves but also their associated circuits. The characteristics or properties of the rectifying elements which will affect the rectification losses are divided in general into the static or steady-state characteristics or properties such as the forward voltage drop, reverse current and so on and the transient characteristics or properties observed during a turn-on time and a turn-off time. The circuit characteristics or properties mainly cause high-frequency losses. Especially when the output voltage of a rectifier is low, the forward voltage drop will adversely affect the rectification efficiency. When the reverse recovery time, which is one of the transient characteristics or properties, becomes comparable with the period of the input voltage, its effects on the rectification efficiency will not remain negligible. As a consequence, when the input voltage or current rapidly changes in level at high frequencies as does a rectangular voltage or current, the adverse effects due to the transient characteristics or properties of a rectifying element used must be taken into consideration.
In the case of a rectifier which rectifies a current at a low frequency and especially at a low voltage such as a commercial power supply, the only requirement for attaining a high rectification efficiency is that the forward voltage drop is low. As a result, germanium diodes and Schottky barrier type diodes are widely used. In the case of the rectification of the high-frequency input current, the transient characteristics must be taken into consideration, so that the Schottky barrier type diodes and fast-recovery diodes are used. However, rectifying elements with a reverse recovery time of less than 50 ns are not available at present, so that it is extremely difficult to attain satisfactorily high efficiency at high frequencies. With respect to the forward voltage drop and the turn-off characteristics, the Schottky barrier type diodes are advantageous over the fast-recovery type diodes, but the former have a low breakdown voltage so that they can be used only when the input voltage is low. The forward voltage drop of these rectifying elements is, in general, of the order of 0.5 V at their maximum rated current. Even when they are used at 1/10 or 1/100 of the maximum rated current, the forward voltage drop is higher than 0.25 or 0.2 V, respectively. Because of the above-described characteristics of these rectifying elements, the rectification efficiency of the rectifiers and especially those operating at high frequencies is limited.
Mechanical rectifiers have been used as the power rectifiers for rectifying the voltages at relatively low frequencies such as a commercial frequency. Since a set of contacts is used, the mechanical rectifiers are advantageous over the semiconductor rectifying elements in that the forward voltage drop is low, but it is difficult to synchronize the closing and opening of the contacts with the zero crossing of an AC voltage, so that the operation is not stable especially at high frequencies. Thus, the mechanical rectifiers operating at high frequencies are not satisfactory in practice.
In the phase detectors (such as the phase discriminators, phase sensitive amplifiers, or lock-in amplifiers) used in communications equipment and measuring instruments, there have been used synchronized rectifiers utilizing the field-effect transistors or bipolar transistors and the combinations of synchronized circuits and amplifiers. However, synchronized rectifiers utilizing the semiconductor active elements have not been used as power rectifiers. It may be considered that the use of field-effect transistors and bipolar transistors as the rectifying elements is advantageous or optimum, because they have a low forward voltage drop, a high reverse resistance and excellent transient characteristics. However, in practice, when they are utilized in the synchronized power rectifiers, it is difficult to achieve a desired phase relationship between the input voltage and the control voltage because the load current, load resistance and load impedance are not necessarily maintained uniform. Especially, when the load is of the type having a capacitance or producing the counter electromotive force, the rectified current flows due to the instantaneous difference between the input and load voltages, so that an excessive reverse current flows into or an excessive overvoltage is applied to a semiconductor active element which is turned on. As a result, the active element is damaged. In the case of a circuit with a high interior impedance such as a phase detector for discriminating the phase by utilizing the reverse current which flows when the semiconductor active element is turned on, the active element can be utilized as a rectifying element in a relatively simple manner, but it is very difficult to utilize the semiconductor active elements as the rectifying elements in the power rectifiers in order to attain a higher degree of efficiency.